1. Field of the Invention
This invention relates to a length measuring instrument comprising a scale member, which is provided with a scale, and a scanning unit, which is movable along the scale and provided with signal-generating means, which are accommodated in at least one covering hood, which is open toward the scale member and together with the scale member defines at least one measuring chamber and is provided with elastomeric scraping means in contact with the scale member.
More particularly, the invention relates to incremental length measuring instruments for an optoelectronic, capacitive, inductive or magnetic scanning of a suitably designed scale member.
2. Description of the Prior Art
Length measuring instruments of the present kind are mounted, e.g., on machine tools and may be used for a normal measurement of lengths or may be included in control systems for machine tools or industrial robots.
A length measuring instrument of the kind described first hereinbefore has been disclosed in U.S. Pat. No. 4,400,890 and comprises a scale member consisting of permanent magnets having a length of at least 2 mm each. Two spaced apart detecting heads are provided and generate signals in response to the movement of the scanning unit or detecting head body along the scale. The scale member is mounted on the bed of a machine tool and has an exposed top surface. The covering hood or case has the same width as the scale member and bears on the scale member with scrapers, which are attached to the ends of the hood and extend transversely to the scale. The scrapers serve to remove coarse impurities, particularly particles of low-reluctance material, from the scale member.
This known measuring instrument can be used only for coarse measurements and does not permit a measurement with high accuracy.
For more accurate measurements, incremental scales having much smaller increments are arranged to cooperate with suitable scanning units. Such length measuring instruments for an optoelectronic scanning have been disclosed in U.S. Pat. No. 3,816,003 and European Pat. No. 2,0120 205. They comprise a scale having small increments and are provided with electronic circuitry including bridge or potentiometer circuits or interpolating computers for effecting an electronic subdivision of the scale increments by a processing of the phase-displaced signals generated in response to the scanning of the scale. The higher the resolution provided by the length measuring instrument, the more susceptible will the instrument be to errors due to a contamination of the scale, a condensation of droplets on the scale and a similar contamination on the scanning unit. For this reason such length measuring instruments usually comprise a tubular housing, in which the scale member, consisting in most cases of glass, and the scanning unit are encapsulated, and the scanning unit is connected to a sword, which extends between pairs of sealing lips through a slot formed in the housing and can be operated to move the scanning unit along the scale. If the scale is to be optically scanned, this can be effected with light transmitted through the scale member from light sources disposed on one side of the scale member and moved along the scale by the scanning unit, which comprises light receivers disposed on the other side of the scale member. Alternatively, scanning can be effected with reflected light if the light sources and receivers are disposed on the same side of the scale member, which may be provided with a reflecting coating on the opposite side.
In order to reduce the risk of contamination or a condensation of moisture, U.S. Pat. No. 3,816,003 to provide in the slot of the housing two pairs of sealing lips, which pairs are arranged one over the other and cooperate with the sword. But in that known arrangement the gaps formed between said sealing lips by the sword have vertically aligned end portions so that moisture and dust can enter the interior of the housing.
European Pat. No. 2,0120, 205 discloses for a scanning with transmitted light an instrument in which the light sources and the light receivers are encapsulated in separate chambers and discloses for a scanning with reflected light an instrument in which the light sources and the light receivers are accommodated in a common chamber, which is defined by a window of transparent material, which comprises a reticle for scanning the scale and is moved along the scale member in close proximity thereto. In either case, the guiding means are disposed in separate parts of the encapsulating housing. The gap between the window or the scanning reticle and the scale member should be so small that any droplets condensed on the scale member will be transformed into a liquid film between the window and the scale member. Although that arrangement is highly expensive, it does not reliably prevent a contamination of the scale member and an occurrence of measurement errors resulting from condensed droplets.